The present invention relates to an antenna device, in particular, to a compact antenna device suitable for receiving radio waves from an artificial satellite as well as ground waves.
Antenna devices using a GPS (global positioning system) have been well known as antenna devices for receiving radio waves from an artificial satellite (hereinafter referred to as satellite waves). This type of GPS antenna devices for vehicle-mounted communication terminals or mobile communication terminals are required to be miniaturized.
An example of the GPS antenna devices will be described with reference to FIG. 1. FIG. 1 shows an example of a compact flat patch antenna device. An antenna device 30 includes a dielectric block 31 comprising resin or a ceramic material and a radiation element 32 disposed on the top surface of the dielectric block 31. A ground conductor 33 is disposed on the bottom surface of the dielectric block 31. Further, a grounding substrate 34 is disposed under the bottom surface of the dielectric block 31 and is electrically connected to the ground conductor 33. This type of antenna device is disclosed in Japanese Patent Application Publication (JP-A) No. 2002-198725, for example.
Although not shown in the figure, a feeding point is usually set in the radiation element 32 in this type of antenna device. Further, a feeding conductor (not shown) is connected to the feeding point through a through-hole (not shown) provided in the dielectric block 31, the ground conductor 33, and the grounding substrate 34. The feeding conductor is derived from the bottom side of the grounding substrate 34.
FIGS. 2A and 2B show the matching frequency and vertical radiation pattern characteristics of the antenna device shown in FIG. 1. The matching frequency is 1.927 (GHz) and the gain is 2.372 (dBi).
Herein, the frequency of a satellite wave transmitted from a GPS satellite is about 1.575 (GHz). Therefore, by shifting the matching frequency of the antenna device having the characteristics shown in FIGS. 2A and 2B to the vicinity of 1.57 (GHz), a receiving characteristic for the satellite wave can be enhanced. In the conventional antenna devices, a dielectric material of high permittivity is used for a dielectric block or the size of dielectric block is set large in order to obtain a lower matching frequency.
However, the cost increases by using a high-permittivity material. On the other hand, by setting the size of dielectric block large, the entire antenna device also becomes large. This is against the requirement for miniaturization.